1. Field of the Invention
The present invention relates to a process for manufacturing electrical cables, in particular electrical cables for power transmission or distribution at medium or high voltage.
In the present description, the term medium voltage is used to refer to a tension typically from about 1 kV to about 60 kV and the term high voltage refers to a tension above 60 kV (very high voltage is also sometimes used in the art to define voltages greater than about 150 kV or 220 kV, up to 500 kV or more).
Said cables may be used for both direct current (DC) or alternating current (AC) transmission or distribution.
2. Description of the Related Art
Cables for power transmission or distribution at medium or high voltage generally have a metal conductor which is surrounded, respectively, with a first inner semiconductive layer, an insulating layer and an outer semiconductive layer. In the following of the present description, said group of elements will be indicated with the term of “core”.
In a position radially external to said core, the cable is provided with a metal shield (or screen), usually of aluminium, lead or copper.
The metal shield may consist of a number of metal wires or tapes, helically wound around the core, or of a circumferentially continuous tube, such as a metallic tape shaped according to a tubular form and welded or sealed to ensure hermeticity.
The metal shield performs an electrical function by creating, inside the cable, as a result of direct contact between the metal shield and the outer semiconductive layer of the core, a uniform electrical field of the radial type, at the same time cancelling the external electrical field of the cable. A further function is that of withstanding short-circuit currents.
When made in circumferentially continuous tubular form, the metal shield also provides hermeticity against water penetration in the radial direction.
An example of metal shields is described in U.S. Re36,307.
In a configuration of the unipolar type, said cable further comprises a polymeric oversheath in a position radially external to the metal shield mentioned above.
Moreover, cables for power transmission or distribution are generally provided with one or more layers for protecting said cables from accidental impacts which may occur on their external surface.
Accidental impacts on a cable may occur, for example, during transport thereof or during the laying step of the cable in a trench dug into the soil. Said accidental impacts may cause a series of structural damages to the cable, including deformation of the insulating layer and detachment of the insulating layer from the semiconductive layers, damages which may cause variations in the electrical voltage stress of the insulating layer with a consequent decrease in the insulating capacity of said layer.
Cross-linked insulation cables are known and their manufacturing process is described, for example, in EP1288218, EP426073, US2002/0143114, and U.S. Pat. No. 4,469,539.
The cross-linking of the cable insulation can be made either by using the so-called silane cross-linking or by using peroxides.
In the first case, the cable core, comprising the extruded insulation surrounding the conductor, is maintained for a relatively long period of time (hours or days) in a water-containing ambient (either liquid or vapor, such as ambient humidity), such that the water can diffuse through the insulation to cause the cross-linking to take place. This requires the cable core to be coiled on spools of fixed length, fact which inherently prevents a continuous process to be carried out.
In the second case, the cross-linking is caused by the decomposition of a peroxide, at relatively high temperature and pressure. The chemical reactions that take place generate gaseous byproducts which must be allowed to diffuse through the insulation layer not only during the curing time but also after the curing. Therefore a degassing step has to be provided during which the cable core is stored for a period of time sufficient to eliminate such gaseous byproducts before further layers are applied over the cable core (in particular in case such layers are gas-tight or substantially gas-tight, such as in the case a longitudinally folded metal layer is applied).
In the practical experience of the Applicant, in the absence of a degassing stage prior to further layers application, it may happen that under particular environmental conditions (e.g. remarkable solar irradiation of the cable core) said byproducts expands thus causing undesired deformations of the metal shield and/or of the polymeric oversheath.
Furthermore, in the case a degassing step is not provided, the gaseous byproducts (e.g. methane, acetophenone, cuminic alcohol) remain trapped within the cable core due to the presence of the further layers applied thereto and can exit the cable only from the ends thereof. This is particularly dangerous since some of said byproducts (e.g., the methane) are inflammable and thus explosions may occur, for instance during laying or joining of said cables in the trench dug into the soil.
Furthermore, in the absence of a degassing stage prior to further layers application, it may happen that porosity in the insulation is found which can deteriorate the insulation electric properties.
A process for producing a cable having thermoplastic insulation is described in WO02/47092, in the name of the same Applicant, where a cable is produced by extruding and passing through a static mixer a thermoplastic material, comprising a thermoplastic polymer mixed with a dielectric liquid, such thermoplastic material being applied around a conductor by means of an extrusion head. After a cooling and a drying step, the cable core is stored on a reel and then a metal shield is applied by helically placing thin strips of copper or copper wires onto the cable core. An outer polymer sheath then completes the cable.
The continuous supply of the cable core with extruded insulation to the shield application unit was not contemplated. In fact the shield was of a type only suitable for a non-continuous application process since it required the use of spools mounted on a rotating apparatus, as further explained in the following.